Diplomats grappled to secure a landmark deal at talks on Iran's disputed nuclear programme on Saturday after Tehran raised the prospect of negotiations being put off. In a rush of late-night diplomacy, Iranian Foreign Minister Mohammad Javad Zarif had joined foreign ministers from six world powers for a session expected to be the last before talks break up. At stake was a proposed deal that could see Iran freeze parts its nuclear programme in exchange for the easing of some of the sanctions that are crippling its economy, as a first step ahead of more talks. US and European officials said Zarif had joined a meeting attended by EU diplomatic chief Catherine Ashton and ministers from the P5+1 group that includes Britain, China, France, Germany, Russia and the United States.